1. Field of the Invention
The invention relates to a circuit board and a circuit apparatus using the same. In particular, the invention relates to a circuit board having a substrate consisting primarily of metal as a core member, and a circuit apparatus using the same.
2. Description of the Related Art
While portable electronics equipment including cellular phones, PDAs, DVCs, and DSCs are rapidly becoming more functionally sophisticated, miniaturization and weight savings have become essential in order for these products to be accepted in the market. In order to achieve these aims, system LSIs offering higher integration have been sought. Meanwhile, electronic equipment with enhanced usability and convenience have been desired, and a higher level of functional and performance sophistication has been required of the LSIs intended for use in such equipment. Consequently, while the LSI chips providing higher integration have enabled a greater numbers of I/Os to be implemented, miniaturization of the packages themselves has been highly sought after. For the sake of satisfying both of these requirements, there has been strong demand to develop a semiconductor package suited to packaging semiconductor parts on a board at a higher density. To meet this demand, various types of packaging technologies called CSP (Chip Size Package) have been developed.
Among the known examples of such packages is a BGA (Ball Grid Array). With the BGA, semiconductor chips are mounted on the packaging board and molded with resin. Then, solder balls are formed over an area on the opposing side of the board as external terminals. Since the BGA utilizes a planar mounting area, it is relatively easy to miniaturize the package. Further to this, the circuit board need not be rendered in narrower pitches and does not require high-precision mounting technologies. The BGA can thus be used to reduce the total packaging cost, even if the package itself is of a relatively high cost.
FIG. 17 is a diagram showing the general configuration of a typical BGA. The BGA 100 has a structure where an LSI chip 102 is mounted on a glass epoxy board 106 via an adhesive layer 108. The LSI chip 102 is molded within a sealing resin 110. The LSI chip 102 and the glass epoxy board 106 are electrically connected by metal wires 104. Solder balls 112 are arranged in an array on the back side of the glass epoxy board 106. Through these solder balls 112, the BGA 100 is mounted on a printed wiring board.
In recent years, semiconductor packages (circuit apparatuses) for incorporation into electronic equipment and the like have required increased miniaturization, higher densities, and more functionality. The circuit apparatuses have thus increased the heat generation density per unit volume. Such increases in the heat generation density can have an adverse effect on the performance and reliability of the circuit apparatuses, thereby causing significant problems. For this reason, metal substrates and the like having high radiation performances have been used as circuit boards for constituting the circuit apparatuses as an alternative to the glass epoxy board 106.
FIG. 18 is a sectional schematic view showing another structure of a conventional circuit apparatus as disclosed in Japanese Patent Laid-Open Publication No. 2002-335057. With reference to FIG. 18, the circuit board 210 for constituting a conventional circuit apparatus 200 includes a metal substrate 201 as a core member. Wiring pattern layers 203 and 205 are formed on both sides of the metal substrate 201 via resin insulating layers 202 and 204. Through apertures called through holes 206 are formed in the direction of thickness to provide electrical connection between the layers. The inner surfaces of the through holes are plated with copper or the like, thereby forming an electrical conduction layer 207 for layer-to-layer conduction. Further to this, a semiconductor chip 220 is directly connected to one side of the circuit board 210 via solder balls 221.
In general, the metal substrate 201 is made of a metal material that requires a low coefficient of thermal expansion. Conversely, the insulating layer 202 is made of a resin having a high coefficient of thermal expansion. For example, the metal substrate is made of an Fe—Ni—Co alloy having a coefficient of thermal expansion of 10×10−6/K, or copper (Cu) having a coefficient of thermal expansion of 6×10−6 to 9×10−6/K. Meanwhile, the insulating film is made of an epoxy resin having a coefficient of thermal expansion of 60×10−6 to 70×10−6/K. That is, the insulating film has a coefficient of thermal expansion approximately ten times that of the metal substrate. Due to such a difference in the thermal expansion property, the wiring pattern layer 203 formed on the insulating layer 202 undergoes displacement as the circuit board 210 rises in temperature. FIG. 19 is a sectional view of the circuit board in which displacement occurs due to a rise in temperature. In the diagram, an electric conductive layer 207a, having no thermal-expansion displacement, is shown by the broken line. As shown in FIG. 19, the distance as much as which the wiring pattern layer 203 formed on the insulating layer 202 is shifted by expansion (the amount of displacement caused by the expansion) is greater than that of the electrical conduction layer 207 which is formed inside the through hole 206 of the metal substrate 201. The two patterns thus cause a relative displacement with respect to one another. If this displacement is repeated due to temperature variations in the circuit board 210, cracks can sometimes occur between the wiring pattern layer 203 and the electrical conduction layer 207. It therefore follows that this portion makes a poor connection, resulting in reduced reliability. Moreover, since the metal substrate 201 and the insulating layer 202 are bonded only by the adhesion of the contact surfaces, problems such as film exfoliation from the metal substrate 201 may occur, depending on the amount of displacement of the expanding insulating layer 202. As a result, the circuit board 210 and the circuit apparatus using the same may bring about deterioration in reliability.
In addition to this, the wiring material used for the circuit board 210 (for example, copper) is typically under compression stress (stress in the direction in which the wiring material contracts), which occurs in the process of formation. Even after the wiring material is patterned into the desired wiring pattern layers 203 and 205, the wiring pattern layers 203 and 205 still have compression stresses (the stresses A and B). The higher wiring densities the wiring pattern layers have, the higher the remaining compression stresses are. As shown in FIG. 18, if the wiring pattern layer 203 formed on one side of the metal substrate 201 has a wiring density higher than that of the wiring pattern layer 205 formed on the other side, the circuit board 210 can be affected by the compression stress acting on the side of the wiring pattern layer 203 (the differential compression stress between the stresses A and B).
FIG. 20 is a sectional view of a circuit apparatus having a deformed circuit board. FIG. 20 shows a case where the compression stress acting on the side of the wiring pattern layer 203 (the differential compression stress between the stresses A and B of FIG. 18) exceeds the rigidity of the circuit board 210 (in particular, the metal substrate 201). Here, the circuit board 210 contracts to warp and deform toward the side of the wiring pattern layer 203. Specifically, the wiring pattern layer 203 and the resin insulating layer 202 undergo a compression stress (the stress in the direction in which the materials contract). The wiring pattern layer 205 and the resin insulating layer 204 on the other side undergo a tensile stress (the stress in the direction in which the materials expand). Consequently, the wiring pattern layers 203 and 205 can easily cause migration of the wiring material, with a resultant drop in the wiring reliability. The resin insulating layers 202 and 204, on the other hand, may possibly cause exfoliation from the metal substrate 201 and the wiring pattern layers. As a whole, the circuit apparatus 200 having the circuit board 210 is susceptible to deterioration in reliability.